Referring to FIG. 1, FIG. 1 illustrates a cross-sectional view of a conventional organic light-emitting diode. A conventional organic light-emitting diode 100 includes a transparent substrate 102. An anode 104 is formed on the substrate 102, and a hole transport layer (HTL) 106 and an electron transport layer (ETL) 108 are formed to stack on the anode 104 in sequence. A cathode 110 is then formed on the electron transport layer 108. A carrier transport layer composed of the hole transport layer 106 and the electron transport layer 108 is located between the anode 104 and the cathode 110 for transporting holes and electrons. The electron transport layer 108 is usually called an illuminant layer.
In an organic material layer, the hole is about ten times as mobile as the electron, and the interfacial energy barrier faced by the hole is smaller than that faced by the electron, so that when large number of holes inject into the electron transport layer 108, only a few electrons reach the electron transport layer 108, resulting in the amount unbalance of the electrons and the holes. A large number of holes cannot be combined with electrons, and thus the luminance current efficiency and the luminous flux power efficiency of the organic light-emitting diode 100 are reduced.
Presently, the research of organic light-emitting diode in promoting luminance efficiency generally gives more emphasis on improving the injection efficiency of electrons and heavy doping the electron transport layer with organic impurities. In the aspect of improving the injection efficiency of electrons, an alloy layer of low work function is added between a cathode and an electron transport layer to increase the number of injected and transported electrons after the electron transport layer made of organic material is formed, so as to achieve the objective of improving the injection efficiency of electrons. In the aspect of heavy doping the electron transport layer with organic impurities, organic impurities, such as tetraphenylporphyrin (TPP), are doped into the electron transport layer, and the organic impurities are used to catch excitations and to block these excitations from moving to the cathode and being eliminated. Therefore, the excitations falls to a steady state energy level from an exciting state to increase chances for emitting light, so as to achieve an objective of enhancing luminance efficiency of the organic light-emitting diode.
However, although the luminance efficiency of an organic light-emitting diode can be enhanced with an alloy layer of low work function added between a cathode and an electron transport layer, yet the number of process steps is increased and process is more complicated, thus increasing the process cost. Additionally, although the luminance efficiency of an organic light-emitting diode can also be enhanced by heavy doping the electron transport layer with organic impurities, yet the organic impurities, such as tetraphenylporphyrin, are very expensive, thus greatly increasing process cost. Therefore, those two methods for improving luminance efficiency of the organic light-emitting diode mentioned above are not benefit for real production.